This invention relates generally to electronic circuitry and more particularly to electronic digital output buffers.
Digital buffers are used to provide buffered logic levels at relatively high output currents to drive external digital devices. In metal-oxide semiconductor field effect transistor (MOSFET) and complementary metal-oxide semiconductor (CMOS) digital circuitry, both the inputs and the outputs to a digital buffer are typically in the 0-5 volt d.c. range, where 0 volts (ground) represents a LO logic state and 5 volts represents a HI logic state. A type of digital buffer having a third, high impedance output state is known as a tri-state buffer. A digital output buffer often performs an inverting function so that the output of the buffer is the opposite logic state as the input to the buffer.
Digital electronic circuits typically include data, control, and address busses as well as individual data lines which carry information throughout the circuitry. Many tri-state buffers are typically coupled to each of the data and bus lines. Ideally, only one buffer connected to a data or bus line should be active at a particular time, to prevent contention among the buffers on that line. In other words, when a buffer coupled to a data or bus line is active, i.e. trying to drive that line HI or LO, all of the other output buffers coupled to that line should be in their high-impedance or tri-state.
Unfortunately, an output buffer is sometimes activated before a previously activated output buffer has been tri-stated, causing output contention on the data or bus line. If, for example, an activated output buffer is attempting to drive a line HI, while the previously activated output buffer is still pulling the line LO, large currents will flow through the activated output buffer, through the bus line, and through the previously activated output buffer. If the newly activated output buffer is attempting to pull the bus line LO, and if a previously activated buffer is still pulling the bus line HI, excessive currents will likewise flow through the two buffers via the common data or bus line.
This output contention situation not only causes excessive power consumption by the digital electronic circuitry but it can also cause a subsequent Ldi/dt power supply disturbance as the large current caused by the contention abruptly ceases when the previously activated buffer tri-states. This power supply disturbance can cause transients within the circuit, possibly disturbing the performance or even damaging components of the circuitry.
The prior art, therefore, does not adequately address the problem of output contention between the digital output buffers of a digital electronic circuit.